Investigation of a Circularly Polarized Log-Periodic Dipole Antenna

In this letter, a circularly-polarized (CP) log-periodic dipole antenna (LPDA) is investigated. Eight crossed dipole cells with a logarithmically increased $\lambda $ /8 separation are employed to form the proposed CP LPDA. The dipoles with increased length and spacing are fed in series by an air-filled parallel line. The working mechanism of the generation of back-fire CP radiation is illustrated by an array analyzation. Besides, the LPDA design factors for both polarizations are analyzed in detail to realize an optimized CP performance. To validate the performance of the proposed eight-element CP LPDA, a prototype was fabricated and measured. The measured axial ratio (AR) bandwidth (0.8-2.5 GHz) is coincident with the impedance bandwidth (0.8-2.43 GHz), and it can be further increased with more elements. The overlapping bandwidth with VSWR < 2, AR < 3 dB, gain variation < 3 dB, and back-fire gain > 5 dBic is 0.94-2.43 GHz (2.6:1), and the radiation pattern is stable across the entire band.